The invention is in the field of digital image processing and is directed to a method for converting image data of an image onto a different coordinate system, for example onto a different size or onto a different resolution or the calculation of a rotated or perspectively distorted image. Coordinate conversions for image data are employed in all fields of employment of digital image processing.
Electronic reproduction technology is an important field of employment, whereby extremely large sets of image data are processed. In reproduction technology, print masters are produced for printing pages that contain all elements to be printed such as texts, graphics and images. In the case of electronic production of print masters, these elements are present in the form of digital data. The data are generated for an image, for example, in that the image is scanned point-by-point and line-by-line in a scanner, each picture element is resolved into color components, and the color values of these components are digitalized. Images are usually resolved into the color components red, green and blue (RGB) in a scanner. For four-color printing these components are then transformed further into the printing inks cyan, magenta, yellow and black (CMYK).
During the course of further work, the digitalized texts, graphics and images are assembled in a processing station to form a printing page either under visual control on a color monitor or automatically according to stored layout rules. The printing page is then written in a data format, for example in the page description language PostScript, and is stored. In order to produce a printing plate for each printing ink, the printing page data are composed of color separation data for respectively one of the printing inks CMYK. The color separation data are recorded in extremely high resolution on film material or directly on printing plates with a film or, respectively, plate recorder. There are also digital printing machines that work without printing plates. In this case, the color separation data are transmitted directly to the digital printing machine and are printed out thereat on the material to be printed.
For recording on film material or printing plates, the color separation data are converted into high-resolution bit map data in a raster image processor (RIP), whereby screen dots of different size are generated for the different density values of the colors. The result is a high-resolution, binary image that now has only two brightness values per picture element (black or, respectively, white). In English-language usage, such an image is also referred to as a “bit map” and is also referred to as “Strichbild” in German usage.
A resolution for images that is standard in reproduction technology is, for example, twelve picture elements/mm (300 picture elements/inch). When four color values are stored (CMYK) per picture element and the densities of the color values are digitized with respectively eight bits (equal one byte), then a data set of210 mm×297 mm×12×12×4=35,925,120 bytes  (1)derives from an image having a size of DIN A4, i.e. approximately 36 megabytes. The bit map data are produced in a significantly higher resolution in the raster image processor, for example with 100 picture elements/mm (2540 picture element/inch). However, each picture element is digitalized with one bit since it can only assume two brightness values. A data set of210 mm×297 mm×100×100×⅛×4=311,850,000 bytes  (2)thus derives for the four color separation data of a DIN A4 printing page, i.e. approximately 312 megabytes. These datasets are also multiplied by the number of pages in a printed product. Extremely large data sets must thus be processed in electronic reproduction technology. The employment of effective processing methods therefore plays a great part.
It often occurs in reproduction technology that digital image data must be converted onto a different image size (upon retention of the resolution) or onto a different resolution (upon retention of the image size). The same problem is to be solved in both instances of scale conversion. Using a given number of picture elements per image line and a given number of picture lines, a different number of picture elements per image line and a different number of image lines must be calculated. The color values of the newly calculated picture elements must be defined from the corresponding picture elements of the original image such that the image quality of the converted image does not suffer. The methods known from the prior art are extremely time-consuming for the large image data sets in reproduction technology. Original image structures can be falsified in the converted image and disturbing patterns (Moirè) can also arise, specifically in the scale conversion of screened bit map data.